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Date: Thu, 2 May 2024 20:48:34 +0200
From: Robert Richter <rrichter@amd.com>
To: Yazen Ghannam <yazen.ghannam@amd.com>
Cc: Borislav Petkov <bp@alien8.de>, linux-edac@vger.kernel.org,
	linux-kernel@vger.kernel.org, tony.luck@intel.com, x86@kernel.org,
	Avadhut.Naik@amd.com, John.Allen@amd.com
Subject: Re: [PATCH v2 07/16] x86/mce/amd: Simplify DFR handler setup
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On 02.05.24 12:02:02, Yazen Ghannam wrote:
> On 4/30/24 2:06 PM, Borislav Petkov wrote:
> > On Mon, Apr 29, 2024 at 08:34:37PM +0200, Robert Richter wrote:
> > > After looking a while into it I think the issue was the following:
> > > 
> > > IBS offset was not enabled by firmware, but MCE already was (due to
> > > earlier setup). And mce was (maybe) not on all cpus and only one cpu
> > > per socket enabled. The IBS vector should be enabled on all cpus. Now
> > > firmware allocated offset 1 for mce (instead of offset 0 as for
> > > k8). This caused the hardcoded value (offset 1 for IBS) to be already
> > > taken. Also, hardcoded values couldn't be used at all as this would
> > > have not been worked on k8 (for mce). Another issue was to find the
> > > next free offset as you couldn't examine just the current cpu. So even
> > > if the offset on the current was available, another cpu might have
> > > that offset already in use. Yet another problem was that programmed
> > > offsets for mce and ibs overlapped each other and the kernel had to
> > > reassign them (the ibs offset).
> > > 
> > > I hope a remember correctly here with all details.
> > 
> > I think you're remembering it correct because after I read this, a very
> > very old and dormant brain cell did light up in my head and said, oh
> > yeah, that definitely rings a bell!
> > 
> > :-P
> > 
> > Yazen, this is the type of mess I was talking about.
> > 
> 
> Yep, I see what you mean. Definitely a pain :/
> 
> So is this the only known issue? And was it encountered in production
> systems? Were/are people using IBS on K8 (Family Fh) systems? I know
> that perf got support at this time, but do people still use it?
> 
> Just as an example, this project has Family 10h as the earliest supported.
> https://github.com/jlgreathouse/AMD_IBS_Toolkit

No, IBS was introduced with 10h, but the eilvt offset came already
with k8, but with only one entry. That affected productions systems
and BIOSes.

> 
> My thinking is that we can simplify the code if there are no practical
> issues. And we can address any reported issues as they come.
> 
> If you think that's okay, then I can continue with this particular clean
> up. If not, then at least we have some more context here.

The general approach to use the preprogrammed offsets looks good to
me. Though, existing code [1] checks the offset and reapplies a
hardcoded value of 2 if it is zero. I don't know the history of
this. However, it would be great if that code could be simplified.

-Robert

[1] commit 24fd78a81f6d ("x86/mce/amd: Introduce deferred error interrupt handler")

> 
> I'm sure there will be more topics like this when redoing the MCA init path.
> 
> :)
> 
> Thanks,
> Yazen